Plenty Plenum Confusion

Know your setup – including the chemical processes involved – and you’ll save yourself a lot of unnecessary difficulties.

I recently heard a former saltwater aquarist ask about his plenum setup – in his new freshwater planted aquarium.

This is somewhat odd for a few reasons. To understand why I think this, I’ll have to give a little background on the plenum system.

Plenum Methods
The plenum method is based on the work of professor Jean Jaubert, who’s worked laid the way for the use of plenums in reef aquaria. Professor Jaubert’s systems were more complex than simply incorporating a plenum into a captive marine aquarium environment, and we won’t go into all of the specifics here, but we will take a general look at how plenums are used in marine aquaria. Reef Aquarium Checklist>>

Plenums have been popular in saltwater aquaria for a while now, though they’ve recently fallen out of favor as new methods have gained acceptance.

In a marine aquarium, a plenum is just an open area of water underneath a sandbed that has been separated from the sand above by a grate and a fine mesh screen. The main point of a plenum is to create a near-anaerobic (low oxygen) area beneath a deep (4 to 5 inch) sandbed where specific bacteria that break nitrates down into nitrogen gas can thrive.

This allows for the export of nitrates from the aquarium. Sounds good, right? The plenum system was popular for a while in the marine hobby, and is still widely used by reef aquarists, but the method has fallen out of vogue for a few reasons.

The main concern is that some areas of the plenum will develop true anaerobic conditions without any oxygen. In this case, excess organic waste may be converted to hydrogen sulfide, which is toxic and has the potential to “tank” the tank, so to speak.

The other concern has to do with the idea that eventually, the system’s efficiency will be compromised in the long term as excess detritus accumulates, but this hasn’t been conclusively proven yet. Also, there are various design considerations that may play a role in a marine plenum’s long-term viability.

This is a very simplified description of a marine plenum system. There are a lot of other considerations to keep in mind if one is trying to establish an effective plenum system, but in my opinion, if a plenum in a marine aquarium is properly designed and maintained, it can offer effective biological filtration for a long time.

However, in a freshwater planted aquarium, I doubt the efficacy of including a plenum at all.

Problems with Planted Plenums
See, the whole reason for building a plenum is to create near-anaerobic areas beneath a substrate. I think that in a planted tank, low oxygen levels beneath the substrate (unless it were excessively deep) would be impossible to maintain.

To even have the possibility of creating near-anaerobic conditions, we would have to use a very deep substrate. The reason for this is that plant roots will add oxygen to the substrate to create the right environment for their bacteria to process nutrients in the substrate.

As plant roots grow and spread, even deep substrate areas will be oxygenated. This defeats the purpose of building a plenum in the first place. All we would be doing is creating a very deep substrate with a useless open area in our planted aquariums.

Flying on Autopilot
Why bother with this? The conditions we are trying to create in marine aquaria are vastly different from the conditions we are trying to create in planted aquaria. In a reef tank, we don’t have plants (except algae, generally) to absorb excess nutrients. However, in planted aquaria, plants will effectively uptake most nutrients in the system if it is properly designed, maintained and balanced.

I suspect the person asking this question was relying on his knowledge of reefkeeping (which is likely very extensive) to understand how to maintain a planted tank. In this case, I think this aquarist should drop the idea of using a plenum in his planted tank and realign his system with the conditions needed for a planted aquarium in mind.

Using other’s techniques and methods to succeed in the aquarium hobby is a great idea. But we also have to use our heads and understand the processes we are trying to establish in our aquaria.

We can’t rely on canned solutions or “proven techniques” in all circumstances. Even if a given system or technique works for one of us without being modified or even fully understood, it doesn’t mean it will work for all of us.

Education, Not Indoctrination
I bring all of this up because I don’t want to see new hobbyists discouraged by unnecessary failures. New aquarists get excited about the hobby, and rightfully so. They see beautiful, professionally designed aquaria and they want these setups right away.

Naturally, they turn to experienced hobbyists seeking answers and advice. Too often, these answers and advice are taken totally at face value, and systems are set up without an understanding of the goals the aquarists are actually trying to achieve.

Experienced aquarists should definitely give advice and answer new hobbyists’ questions. But I suggest that instead of trying to provide “the answers,” experienced hobbyists should instead seek to educate new aquarists about what they need to learn to be successful.

I want everyone who is serious about keeping aquaria to eventually be their own authorities in the hobby. We can help new aquarists achieve this by educating them, not indoctrinating them.

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